In a resist separation step carried out in related art, a solution called SPM (sulfuric acid-hydrogen peroxide mixture), which is a combination of concentrated sulfuric acid and hydrogen peroxide water, is used. The method consumes a large amount of sulfuric acid and hydrogen peroxide water and is therefore disadvantageous in that the method incurs high running cost and a large amount of liquid waste is produced. To address the disadvantages, the present inventors have already developed a cleaning method and a cleaning system in which electrolyzed sulfuric acid containing an oxidizing substance, such as persulfuric acid, produced by electrolysis of sulfuric acid is used as a cleaning liquid and sulfuric acid is cyclically used (see Patent Literatures 1 and 2, for example). The cleaning system allows reduction in the amount of chemical liquid to be used and the amount of liquid waste to be produced and further provides a high cleaning effect.